Update SA1600 documentation for covariant return types

Author: sharwell

documentation/SA1600.md

@@ -83,6 +83,31 @@ class Car : Vehicle
 }
 ```
 
+Starting with C# 9, overrides can declare covariant return types. Documentation can still be inherited directly from the base member using `<inheritdoc/>`:
+
+```csharp
+/// <summary>
+/// Base factory.
+/// </summary>
+class WidgetFactory
+{
+  /// <summary>
+  /// Creates a widget.
+  /// </summary>
+  /// <returns>The newly created <see cref="Widget"/>.</returns>
+  public virtual Widget Create() => new Widget();
+}
+
+/// <summary>
+/// Specialized factory.
+/// </summary>
+class FancyWidgetFactory : WidgetFactory
+{
+  /// <inheritdoc/>
+  public override FancyWidget Create() => new FancyWidget();
+}
+```
+
 ## How to suppress violations
 
 ```csharp